It's been explained above about the tax changes in 2017, you can also read through the quite complicated banding system on the DVLA's website. Fortunately, my 2017 Nismo sits in the sweet spot of £165 for annual RFL - I doubt that I would have bought it had it cost three times as much, as is the case with those just a year or so older. Take a look through Autotrader and see the vast amount of 2017+ 'performance vehicles' that also cost only £165 to tax.
It's all about getting the older, more polluting, vehicles off the road and getting us into newer ones.
As for MPG and general running costs, the 370Z is a 3.7 V6 (as I'm sure you're aware), so you don't buy it for economy. Mine generally does about 20 mpg locally and around town, but can easily achieve nearly 30mpg on a motorway run - not fantastic, but 50% better than my usual. I did a 365 mile return trip from Surrey to Stafford on the weekend, averaging 27 mpg, a lot of that above 70mph (ahem) on the M40/M6 Toll. If I'd wanted economy on that trip I'd have taken the Ateca which would hit 50mpg, but the Z was more appealing for the job on this occasion!
Lastly, routine servicing is very reasonable, particularly if you use one of the specialist indies. It's a relatively simple and low tech (aka lower cost) car to run and with less to go wrong, say in comparisson with a modern M car for example.